1. Field of the Invention
The present invention relates generally to amplifier circuitry and in particular to an amplifier circuit suitable for implementation in integrated circuit form and having the capability to drive relatively large capacitive loads while maintaining low power consumption.
2. Background Art
Referring to the drawings, FIG. 1 depicts a simplified schematic diagram of a conventional integrated circuit operational amplifier, generally designated by the numeral 10. The amplifier includes a first or input stage 10A, a second or intermediate stage 10B and a third or output stage 10C.
The first stage 10A includes a differentially-connected pair of PNP transistors Q1 and Q2 connected to a common current source I1. The collectors of transistors Q1 and Q2 are connected to an active load comprising NPN transistors Q3 and Q4. Transistors Q3 and Q4 function as a current mirror and provide a single-ended current output at the collector junction of transistors Q4 and Q2.
The current output of the first stage is coupled to the input of the second stage 10B which includes an NPN transistor Q5 connected in a common emitter configuration. A frequency compensation capacitor Cc is connected between the base and collector of transistor Q5 to form the well known Miller integrator circuit. Capacitor Cc functions to provide a dominant pole in the frequency response of the amplifier. A current source 12 in combination with a pair of series connected diodes D1 and D2 form the load for the second stage. The second stage includes a pair of outputs, one at the anode of diode D1 and the other at the cathode of diode D2. Accordingly, the two outputs of the second stage will be that same except they will be offset from one another by two diode drops.
The outputs of the second stage 10B are coupled to the inputs of the third stage 10C. The third stage includes an NPN transistor Q6 and an PNP transistor Q7, both of which are configured as emitter followers. The voltage drops across diodes D1 and D2 are preferably matched to the base-emitter voltages of transistors Q6 and Q7 so that the current flow through the two diodes matches the current flow through the two transistors. In that case, the idle current in the third stage will be controlled by the magnitude of the current source 12 in the second stage.
In many applications, it is necessary for an amplifier to drive relatively large capacitive loads. Output transistor Q6 functions to source current to the capacitive load so that the load becomes charged and transistor Q7 functions to sink current from the load so that the load becomes discharged.
The open loop output impedance of amplifier 10 can be determined by injecting a current signal at the output and measuring the resultant change in output voltage. The lower the change in output voltage, the lower the open loop output impedance. As is well known, the open loop output impedance of the amplifier together with the load capacitance act as a low-pass filter. The "filter" typically introduces a phase lag at the unity gain frequency f.sub.u of the amplifier, i.e. the frequency at which the open loop gain of the amplifier is unity. The phase lag reduces the phase margin of the amplifier causing the transient response to be degraded. Of course, if the phase margin is reduced to zero, the amplifier will become unstable and will oscillate. Stated somewhat differently, the low-pass filter introduces an undesirable secondary pole in addition to the dominant pole produced by capacitor Cc.
One conventional solution to the above-described problem of driving large capacitive loads is to isolate the capacitive load. FIG. 2 shows the amplifier 10 connected to a capacitive load represented by capacitor C1 connected in parallel with resistor RL. A small value resistor R1 is inserted between the output of the amplifier and the load so as to isolate the output from the capacitive load. In addition, a small value capacitor C2 is connected directly from the output of the amplifier 10 to the inverting input of the amplifier. Capacitor C2 and resistor R1 form a frequency compensation circuit.
The main feedback of the FIG. 2 circuit is provided by resistors R2 and R3. Accordingly, small value resistor R1 is contained within the feedback loop and does not substantially reduce the closed loop output impedance of the circuit. At D.C. and low frequencies, the effect of resistor R1 and capacitor C2 can be ignored. However, at higher frequencies, capacitor C2 functions to directly connect the output of amplifier 10 to the inverting input of the amplifier. This effectively prevents the appearance of the phase-lagging signal resulting from the capacitive load at the inverting input at higher frequencies. Stated somewhat differently, capacitor C2 in combination with resistor R1 introduce a zero which functions to cancel out the undesirable secondary pole created by the finite open loop output impedance and the capacitive load. The higher the output impedance, the lower the frequency of the secondary pole and, therefore, the more difficult it is to compensate the pole.
The actual values for resistor R1 and capacitor C2 are usually determined experimentally. Assuming that amplifier 10 is the LF356 integrated circuit amplifier of National Semiconductor, the load capacitance C1 is 0.5 micro Farads and the load resistance is 2 k ohm. The typical value for the isolation resistor is 10 ohms 20 pico Farads for capacitor C2.
The FIG. 2 circuit is typically implemented with amplifier 10 in integrated circuit form and with the remaining components being discrete. The value of the isolation resistor R1 is a function of the open loop output impedance of amplifier 10. The larger the open loop output impedance, the larger the isolation resistor R1. This reduces the frequency of the secondary pole produced by the load capacitor C1 thereby necessitating the use of a larger value of feedback capacitor C2. For example, assuming that the open loop output impedance of amplifier 10 is 10 ohms, the 10 ohm resistor R1 reduces the magnitude of the signal fed back by capacitor C2 by one half. This may provide sufficient phase margin when the amplifier is configured for a closed loop gain of two but may be insufficient for unity gain configurations.
In many applications, it is desirable to implement as much of a particular circuit in integrated circuit form and to minimize the number of components that are in discrete form. However, it is sometimes difficult or impractical to implement certain components in integrated circuit form. For example, it is difficult to build high quality integrated circuit capacitors which do not require a large amount chip area.
Referring back to the amplifier of FIG. 1, capacitor Cc is typically relatively small and can be (and often is) implemented in integrated circuit form. However, capacitor C2 of FIG. 2 is more difficult to implement in integrated circuit form since it is usually much larger in magnitude than Cc.
One solution to the foregoing is to implement everything except capacitor C2 in integrated circuit form. In that event, the integrated circuit package must include two additional pins to accommodate the external discrete capacitor. This is undesirable in many instances, particularly if more than one circuit is to be integrated and placed in a single package. For example, if four amplifier circuits similar to that of FIG. 2 are to be integrated in common and placed in one package (a quad amplifier product), eight additional pins must be added.
A further solution is to lower the open loop output impedance of the amplifier 10. This permits the use of a smaller resistor R1 and capacitor C2. One conventional approach to lowering the output impedance is to increase the quiescent current flow in the third stage 10C (FIG. 1). This can be done in the case of amplifier 10 by designing current source I2 to provide more current. For example, a 10 ohm output impedance requires a bias current in the output stage of somewhat less than 3 mAmps. This results in increased power consumption and attendant heating of the circuit. This also results in lower battery life in the case of battery powered applications. More importantly, the increased heating limits the number of components which may be placed in the integrated circuit since a large number of heat producing components may cause the maximum operating temperature to be exceeded.
The present invention overcomes the above-noted shortcomings of conventional amplifier circuits having improved capacitive drive capability. The output impedance of the basic amplifier is reduced with decreased power consumption. This permits the use of small integrated circuit capacitors (C2) so that external discrete capacitors need not be used. These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following Detailed Description of the Invention together with the drawings.